Billed by Australia’s medicines and therapeutic regulatory agency as the “gold standard for diagnosis of COVID-19”, polymerase chain reaction PCR tests are used globally to identify positive COVID-19 cases.

At the heart of such claims lies the little-understood concept of PCR test cycles and cycle threshold (Ct) values, which are used to identify the presence of COVID-19.

According to Dr Druce, PCR tests for COVID-19 are analysed through “about 40 to 45 heat and cooling cycles”, with most tests in Australia capped at 40 cycles.

“At the end of each cycle, the instrument shines a little laser through the reaction well and takes a reading,” Dr Druce said.

“Slowly, over time, when there’s a sufficient signal in that reaction, you get a glow, like a fluorescence, and that’s plotted on a graph.”

The Ct value of a positive test is the number of cycles it took before the signal was able to be identified, Dr Druce explained, with higher values equating to a weaker positive due to less virus being present in the sample.

“A strong positive sample has a lower Ct value — so something that comes up at cycle 15, for instance, is very strong, extraordinarily strong. Something that comes up at Ct 30 is quite low.”

A person returning a positive PCR test with a Ct value above 30 would be “very unlikely to be infectious”, Dr Druce said, before adding that most positive results were detected in the range of 20 to 30 PCR cycles.

“People being detected in the 30-plus Ct range usually means they’re being detected late, generally because they don’t have symptoms and they’re just being tested arbitrarily,” he explained.

“They don’t know where they are in the disease course because they have no symptoms, which is 40 per cent of COVID-19 patients.”

It was possible, but very rare, that a high Ct value could also mean a person was in the very early stages of their illness and not yet symptomatic.

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